Why would placing the amp on granite cause issues?
In a word, because granite rings. But like everything else it depends on how you use it. Vibration control is as much about shape as material. Tuning forks ring forever not just because of the metal but mostly because of the shape. Your granite being leftover kitchen counter is probably only about an inch or so thick and fairly wide. So you won’t notice it as much. But if you sit it on edge and hit it with something you will hear the sound it makes. Whatever that sound is you will notice is being imparted into the music of whatever component is sitting on it. Sounds crazy but totally true.
Looking at my system what is hard to see is the amp on the granite has a bit of sorbothane between the granite and the BDR. The granite under the turntable is on a sand bed. Both of these are in order to control the granite.
This is a great example of why its so important to understand what you’re doing is not isolating (of which there is no such thing) but vibration control. Even if you somehow had the perfectly isolated shelf or rack or whatever you will still have the problem of the vibration generated in the component itself, which actually turns out to be the majority of the vibration. So ultimately it comes back to vibration control.
Granite is great for mass and stiffness, but it rings and if untreated will impart a hardness to the sound at certain treble frequencies. But the mass is great for putting the music on a solid foundation. Listen to my system you will be astounded at the degree to which this is true. Would never be that way without that 750 lb under the turntable and 150lb slab under the amp.
Tile is even worse than granite. Isolation footers are a misnomer. They’re vibration control pretending to be something other than what they are. Imposters never are quite as good as the real McCoy. The best rack is no rack so the floor is great, just put some BDR Cones and Round Things between the floor and the amps.